Improve Docker image

- Fixes bug in DB volume set in the wrong container.
- Adds persistence for PHP sessions between container recreations.
- Adds production settings to php.ini (error and log settings).

Fixed: pastanagapp:1
Change-Id: Ibfeee630f514f09ec033a9316c37c69441e4fd7a
diff --git a/docker-compose.sample.yml b/docker-compose.sample.yml
index 285d81e..64e090a 100644
--- a/docker-compose.sample.yml
+++ b/docker-compose.sample.yml
@@ -11,7 +11,7 @@
     networks:
       - backend
     volumes:
-      - ./db-data:/var/lib/mysql
+      - phpsessions:/var/phpsessions
     restart: unless-stopped
   db:
     image: mariadb
@@ -20,7 +20,12 @@
       MARIADB_DATABASE: ${DB_NAME}
     networks:
       - backend
+    volumes:
+      - ./db-data:/var/lib/mysql
     restart: unless-stopped
 
 networks:
   backend:
+
+volumes:
+  phpsessions: